A quality condenser should last a very long time .
Here are a couple quotes I borrowed , to answer your question :
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Re: what makes a good condenser go bad? #9
B-racer Jeff Schlemmer
Shakopee, MN, USA USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Oct 23, 2013 07:32 AM
Top Contributor
Joined 11 years ago
16,688 Posts
You want to see .18-.22mF in a condenser, AND a proper bleed-down rate, generally full bleed-down in roughly 5 minutes. If either of those specs is out of tolerance, the condenser will not function properly.
Most condenser failures are caused by poor grounding, which does not allow a condenser to bleed down properly. It holds part of its charge instead of fully discharging, therefor overheating. Worn breaker plate assemblies are a HUGE factor, with moderate to poor to nonexistent grounds. Using 40+ year old engine ground straps is yet another problem. Engine grounding is a game of volume, like a garden hose. If you pump voltage into your ignition, coolant temp and oil pressure sensors, etc... you need an equal flow of electrons out that ground strap. Internal corrosion (not visible) is a HUGE problem and everyone ignores it...
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And :
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Re: what makes a good condenser go bad? #10
george stringe
Frederick, USA USA
1960 MG MGA 1600
Oct 23, 2013 09:41 AM
Joined 5 years ago
29 Posts
The capacitor (condenser) has two functions: 1) it absorbs the back EMF from the magnetic field in the coil to minimize point contact burning and maximize point life; and 2) it forms a resonant circuit with the ignition coil transferring further energy to the secondary side until the energy is exhausted.
When the points open and the magnetic field collapses it also induces a current in the primary as well. It's not very much because there are only a few windings in the primary, but it's enough to jump a small air-gap, such as the one between the just-opening points in the distributor. That tiny spark is enough to erode metal away from the points and you'll 'burn' the points. It prevents the points from arcing and prevents coil insulation breakdown by limiting the rate of voltage rise at the points.
You are unlikely to do anything to cause a condenser failure. A condenser is made by taking two long aluminum foil ribbons and putting an insulator in between them and rolling it up and putting it in a can. One foil is hooked to the can and the other is connected to the wire coming out. The failure is typically either a mechanical failure of the connections inside the condenser or an electrical failure of the insulating material separating the two foils.
Sometimes they do not use an insulating paper or plastic material between the two foils, but use a layer of aluminum oxide coating on the foils. The oxide coating is a nonconductor and is much thinner than any paper or plastic insulator but it can electrically fail or punch through and short the foils when high voltage charge is applied to the two foils in the condenser.
Keeping a known good spare is a good policy.
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Both those quotes from :
https://www.mgexp.com/phorum/read.php?3,2501990